1. Field of the Invention
The present invention is related to a fuel vapor collecting system wherein, in an automobile or the like, a fuel vapor generating from a fuel tank or the like during a stopping of an engine, namely, the vapor of a HC (hydrocarbon) is collected by an adsorbent, purged from the adsorbent using a negative intake pressure when the engine is in operation, and sucked into an intake system.
2. Description of the Prior Art
FIG. 6 is a system diagram of the conventional fuel vapor collecting system described in the Japanese Patent Application Laid-open No. 63-117155 official gazette. A canister 14 contains an adsorbent 16 for high-boiling point HC which mainly adsorbs high-boiling point HC in a fuel vapor, and a canister 15 contains an adsorbent 17 for low-boiling point HC which mainly adsorbs low-boiling point HC. The fuel vapor is introduced into the canister 14 from the upper space of a fuel tank 11 via a fuel vapor passage 18, and the fuel vapor is also introduced from a float chamber 12 via a fuel vapor passage 19. An electromagnetic valve VL.sub.1 is provided in the passage 19.
The canisters 14 and 15 are connected in series by a fuel vapor passage 21, so that the fuel vapor having passed through the adsorbent 16 in the canister 14 is sent to the adsorbent 17 in the canister 15. An electromagnetic valve VL.sub.2 is provided in the passage 21. Fuel vapor purging passages 26 and 27 respectively connected to the canisters 14 and 15 are connected to the stopping of the intake manifold (inspire system) 13, after being joined with a common fuel vapor purging passage 28. To the side of each canister 14, 15 opposite to the side thereof to which the purging passages 26 and 27 are connected, passages 23 and 24 for introducing the air (atmosphere) for purging are connected, respectively. An electromagnetic valve VL.sub.5 is provided in the passage 23.
In the purging passage 28, an electromagnetic valve VL.sub.4 (three-way valve) is provided at the point where the purging passages 26 and 27 meet each other, and an electromagnetic valve VL.sub.3 is provided in the purging passage 26 before it meets with the passage 27. The arrows of dashed line in the figure represent the flows of the fuel vapor when it is purged, and solid lines represent the vapor flows when it is adsorbed.
In the fuel vapor collecting system shown in FIG. 6, while the engine stops, the fuel vapor is introduced into the canister 14 and then further introduced via the passage 21 into the canister 15 where it is adsorbed. After the engine has started, the canisters 14 and 15 to be purged are alternately switched by switching the electromagnetic valves VL.sub.5, VL.sub.4 and VL.sub.3, whereby both of the high- and low-boiling point HC are purged from the respective canisters using the flow of air inspired into the intake system 13.
A large amount of fuel vapor generating during the stopping of the engine has been adsorbed in each canister until the engine starts up. FIG. 7 is a graph showing the relationship between the total amount of the air being intook from the atmosphere introducing passage 23 and having passed through each canister (abscissa) and the amount of fuel purged from the canister (ordinate) when the canister which has adsorbed a large amount of fuel vapor is purged. Since it is considered that the total amount of the intook air is proportional to a purge time, the amount of the purged fuel exponentially decreases with the purge time. In other words, a large amount of fuel vapor is purged immediately after the purge has started.
Thus, immediately after the purge has started in the prior art, a large amount of fuel vapor is purged at once from both canisters 14 and 15 to be supplied to an engine, and consequently the air-fuel ratio in the engine becomes over-rich to adversely affect the driving characteristics. In addition, there is a problem that a long time is taken before the purge is completed, because the fuel vapor is purged only gradually when time has elapsed since the start of the purge and the fuel vapor adsorbed in the canister has decreased. That is, there is a problem that quick purging cannot be performed since the amount of fuel to be purged tends to be excessively large immediately after the starting of the engine, whereas, thereafter, the amount of fuel to be purged promptly becomes smaller.